KVM: PPC: set IN_GUEST_MODE before checking requests

Avoid a race as described in the code comment.

Also remove a related smp_wmb() from booke's kvmppc_prepare_to_enter().
I can't see any reason for it, and the book3s_pr version doesn't have it.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Alexander Graf <agraf@suse.de>